An Organic Light Emitting Diode (OLED) display is a hot topic of the current research of panel display. Compared with a liquid crystal display, OLED has advantages of lower power consumption, lower product cost, self-emitting, a wider angle of view, and a quicker response speed. At present, in a panel display, such as mobile phones, PADs, digital cameras, an OLED display had replaced the traditional Liquid Crystal Display (LCD). The design of the pixel circuit is the core of an OLED, and shall be considered carefully.
Differently from an LCD which uses a stable voltage to control the luminance, an OLED display is driven by a current, and thus needs a stable current to control the luminance. Due to the processing procedure and aging of the devices, the threshold voltage Vth of the driving transistor in the pixel circuit is non-uniform, which causes the currents flowing through each pixel in the OLED display change and thus the luminance of pixels are uneven, and affects the display effect of the whole image. Furthermore, since the current relates to the source of the driving transistor, i.e., the supply voltage, the IR drop will raise difference in the currents at different areas, which causes the luminance of the OLED display is different at different areas.
FIG. 1 shows a diagram of a conventional pixel circuit. As shown in FIG. 1, a pixel circuit of 2 M1C comprises one driving transistor M2, one switch transistor M1 and one storage capacitor Cs. When the scan line Scan selects a row, the scan line Scan inputs a low level signal, and the P-type switch transistor M1 turns on, and the voltage on the data line Data is written into the storage capacitor Cs; when the row has been scanned, the scan line Scan inputs a high level signal, and the P-type switch transistor M1 turns off, the gate voltage stored in the storage capacitor Cs generates a current in the driving transistor M2 to drive the OLED display, so as to ensure that the OLED display is luminous consistently during a frame. The equation of the saturation current of the driving transistor M2 reads as IOLED=K(Vgs−Vth)2. As discussed above, the threshold voltage Vth of the driving transistor M2 will drift due to the processing procedure and aging of the devices, and Vs will be different due to IR drop since the current relates to the supply voltage. Accordingly, the current flowing through each OLED varies as the threshold voltage Vth of the driving transistor and the source voltage VDD of the driving transistor, which causes non-uniformity of the image luminance.